The present invention relates to blind fasteners and, more particularly, to a threaded blind fastener having a positive mechanical locking capability to prevent unthreading.
Threaded fasteners are used in a variety of applications to connect two workpieces together and in other applications. In the construction of aerodynamic designs such as control surfaces on aircraft and the like, a substantially flush surface usually is desired on the accessible side of the panels, while access to the blind side may not be possible. In these cases, the use of a threaded blind fastener is appropriate, since access to only one side of the panel is available to install the fastener.
The use of blind fasteners in the aerospace industry imposes extremely stringent requirements which the fasteners must meet or exceed. For example, the fasteners must securely fasten the panels, yet resist losing their clamping force during the service life of the aircraft. Hence, the fastener must be able to withstand the severe stresses and vibrations imposed upon them by the often harsh environment in which they are used without loosening.
Typical blind fasteners comprise an internally threaded nut body for insertion into aligned holes in the two panels, and an externally threaded cylindrical stem passing in threaded engagement through the nut body. The inserted end of the stem has an enlarged stem head, and the outer end of the stem has a wrench engaging portion. Upon turning motion of the stem relative to the nut body, the stem is moved in an axial outward direction through the nut body. This axial outward movement causes a deformable sleeve around the stem and abutting against the stem head to deform around a tapered nose on the nut body to a fully set condition against the inner panel. The stem further may be provided with a localized weakened region or breakgroove adapted to sever the stem at a predetermined torque. The breakgroove preferably is located axially along the stem such that the stem twists off in substantially flush relation to the outer head of the nut body after the fastener is fully set. The nut body head normally is received in a countersunk, flush relationship to the outer panel, thus providing an aerodynamic surface after the fastener is set.
After the blind fastener has been installed in the panels, stress and vibration will be produced during use of the aircraft, threatening to cause the fastener to loosen and lose its gripping power. Accordingly, some type of locking mechanism must be employed to prevent the stem and nut body from rotating with respect to each other in a loosening direction. In the past, frictional locking mechanisms usually have been used to create a frictional torque lock between the interfering threads of the stem and the nut body. In other cases, positive locking mechanisms have been used, but their applicability has been limited, especially in connection with blind fasteners.
Accordingly, there has existed a definite need for a blind fastener that provides a positive mechanical lock between the stem and nut body of a blind fastener to prevent the fastener from loosening and losing its clamping force when subjected to stress, vibration and other adverse conditions, and which provides a flush surface after the fastener is set. The present invention satisfies this need and provides further related advantages.